1. Field of the Invention
This invention relates to radiation shielding for radioactive materials. More particularly, this invention relates to a shielding composition and container for attenuating gamma rays and absorbing neutrons.
2. Description of the Prior Art
Much effort has gone into developing economical ways to store and finally dispose of increasing amounts of radioactive wastes generated from nuclear power plants and other nuclear facilities, as well as heavy metal sludges from chemical plants. A significant portion of this effort has been directed at improved radiation shielding compositions and containers.
High-level radioactive wastes, including liquids from reprocessing and spent (used) nuclear fuel, typically have half-lives of hundreds of thousands of years. The reprocessing material is generally stored as liquids, then solidified, permanently stored, and disposed of as required. Spent nuclear fuel is stored initially in water cooled pools at the reactor sites awaiting shipment to a permanent disposal site. After about ten years, the fuel can be moved to dry storage containers until such time that the permanent disposal facility becomes available.
Ideal containers for storage and transport of radioactive wastes should confine them safely for at least about 100 years, and preferably about 300 years.
Lead has often been used for gama ray shielding because it is dense, easily worked and relatively inexpensive. Additionally, a lead shield can often be thinner and more compact than a comparable radiation shield made of almost any other material except depleted uranium. This ability to take up less space and be more portable is highly desirable for radiation shielding systems since it is often necessary to move the shielding systems, such as to more remote locations for safety purposes. Additionally, it is often necessary to move the shielding systems, such as to more remote locations for safety purposes. Additionally, it is often desirable to build shielding systems in locations where there is limited space. Since lead tends to accumulate in the body, similar to other heavy-metal poisons, and continues producing toxic effects for many years after exposure it is desirable to eliminate lead from many of its present uses, including radiation shielding, and define substitutes for lead. Efforts have been made to develop radiation shielding systems utilizing depleted uranium (chiefly uranium-238). For example, Takeshima et al., in U.S. Pat. No. 4,868,400, discloses the use of depleted uranium rods or small balls as radiation shielding in an iron cask for shipping and storing spent nuclear fuel.
Due to the radioactivity of uranium, its tendency to corrode and other factors, uranium is usually accompanied by an over coating of anon-radioactive, highly absorbent material, such as steel. For example, in U.S. Pat. No. Re. 29,876, Reese discloses a depleted uranium container, with a corrosion-free coating of stainless steel for transporting radioactive materials. U.S. Pat. No. 5,015,863, Takeshima et al., teaches using depleted uranium particles coated with a metal of high thermal conductivity, such as, aluminum, copper, silver, magnesium, or the like.
Alternative shielding system taught in U.S. Pat. No. 5,334,847, Kronberg teaches a radiation shield having a depleted uranium core for absorbing gamma rays with a bismuth coating for preventing corrosion, and alternatively having a gadolinium sheet positioned between the uranium core and the bismuth coating for absorbing neutrons.
These uranium metal based shielding systems, however, suffer the problem of being relatively expensive. But an even greater difficulty is the avoidance of uranium corrosion and the assurance of the desired long life of the shielding system for spent nuclear fuel. Commercial shielding systems based upon the use of concrete as the shielding material have been developed due to the relatively low cost of concrete relative to metals such as steel, lead and depleted uranium, as well as the ease of casting the material into the desired form in order to assure structural stability it has been necessary to build composite systems such as ones containing a metal liner with a thick concrete outer shell for shielding of the gamma and neutron radiation. Due to these advantages concrete shielding systems now completely dominate the market for shielding of radioactive materials.
However, these concrete systems generally lack mobility or limit the volume of radioactive material that can be stored in a given space due to the great concrete thickness required to obtain the necessary shielding properties. Yoshihisa, in Japanese Patent Document No. 61-091598, does teach the utilization of depleted uranium and uranium oxide aggregate containing concrete for radiation shielding. While this system does have the potential for reducing the thickness of the radiation shielding while maintaining the desired gamma ray penetration factor there are serious problems with this system with degradation of the concrete and obtaining the desired system life of one hundred years, particularly at elevated temperatures. Mechanical properties of the concrete, such as tensile strength and compressive strength, are seriously degraded at elevated temperatures by addition of the uranium aggregate to the concrete.
An attempt at reducing the thickness of a concrete shield while maintaining the desired long life of the container is taught by Suzuki et al., in U.S. Pat. No. 4,687,614. This reference teaches a three layered structure comprising a metallic vessel with a concrete lining as an inner layer which is reinforced with a reinforcing material and strengthened with a polymeric impregnant, and a polymerized and cured impregnant layer as an intermediate layer between the metallic and concrete layers. However, this and like attempts have generally been unsuccessful in achieving the desired size reduction, while maintaining the cost advantages and desired strength and other properties of conventional concrete systems.